[clang][AST] Constexpr evaluator should treat [[gnu::weak]] member pointer comparisons as evaluation failure

[hazohelet]

Summary

This patch fixes the wrong signal from the constexpr evaluator that [[gnu::weak]] member pointer comparison is valid, while it is emitting notes on them.
I found a crashing case fixed by this change and added it as a test case.
https://godbolt.org/z/8391fGjGn

I noticed this while I was working on D146358
CC @rsmith as the original author

[aaron.ballman]

LGTM, though the changes need a release note. Do you still need someone to commit on your behalf? (Alternatively, you could consider requesting commit access: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access)

[aaron.ballman]

clang/test/SemaCXX/crash-lambda-weak-attr.cpp

1

I don't think we need this to be in C++2b mode explicitly, right?

[hazohelet]

Address comments from @aaron.ballman

• Added release note
• Do not specify std c++ version in test code

[hazohelet]

In D148419#4273551, @aaron.ballman wrote:

LGTM, though the changes need a release note. Do you still need someone to commit on your behalf? (Alternatively, you could consider requesting commit access: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access)

Since I don't have commit access, I would like you to commit this on my behalf. Please use "Takuya Shimizu <[e-mail]>" as patch attribution.
I'll try requesting commit access. Thanks!

clang/test/SemaCXX/crash-lambda-weak-attr.cpp

1

Yeah, we don't need it

[hazohelet]

@aaron.ballman
Sorry, this change is breaking the buildbot and because I don't have commit access I would like you to revert it for me.
It seems we need to specify the std option in the test code.

BUILD FAILED: 41435 expected passes 84 expected failures 28291 unsupported tests 1 unexpected failures (failure)

Step 5 (build-unified-tree) warnings: Build unified tree
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/lib/CodeGen/MachineModuleInfo.cpp:73:51: warning: ‘this’ pointer is null [-Wnonnull]

Step 6 (test-build-unified-tree-check-all) failure: 41435 expected passes 84 expected failures 28291 unsupported tests 1 unexpected failures (failure)
******************** TEST 'Clang :: SemaCXX/crash-lambda-weak-attr.cpp' FAILED ********************
Script:
--
: 'RUN: at line 1';   /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/17/include -nostdsysteminc -fsyntax-only -verify /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp
--
Exit Code: 1

Command Output (stderr):
--
error: 'error' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: a lambda expression may not appear inside of a constant expression
error: 'warning' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: 'static_assert' with no message is a C++17 extension
error: 'note' diagnostics expected but not seen: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: comparison against pointer to weak member 'Weak::weak_method' can only be performed at runtime
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: in call to
error: 'note' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: non-literal type '(lambda at /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp:6:15)' cannot be used in a constant expression
5 errors generated.

--

********************

[aaron.ballman]

In D148419#4273901, @hazohelet wrote:

@aaron.ballman
Sorry, this change is breaking the buildbot and because I don't have commit access I would like you to revert it for me.
It seems we need to specify the std option in the test code.

BUILD FAILED: 41435 expected passes 84 expected failures 28291 unsupported tests 1 unexpected failures (failure)

Step 5 (build-unified-tree) warnings: Build unified tree
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/lib/CodeGen/MachineModuleInfo.cpp:73:51: warning: ‘this’ pointer is null [-Wnonnull]

Step 6 (test-build-unified-tree-check-all) failure: 41435 expected passes 84 expected failures 28291 unsupported tests 1 unexpected failures (failure)
******************** TEST 'Clang :: SemaCXX/crash-lambda-weak-attr.cpp' FAILED ********************
Script:
--
: 'RUN: at line 1';   /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/17/include -nostdsysteminc -fsyntax-only -verify /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp
--
Exit Code: 1

Command Output (stderr):
--
error: 'error' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: a lambda expression may not appear inside of a constant expression
error: 'warning' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: 'static_assert' with no message is a C++17 extension
error: 'note' diagnostics expected but not seen: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: comparison against pointer to weak member 'Weak::weak_method' can only be performed at runtime
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: in call to
error: 'note' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp Line 6: non-literal type '(lambda at /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaCXX/crash-lambda-weak-attr.cpp:6:15)' cannot be used in a constant expression
5 errors generated.

--

********************

I noticed that as well and corrected the issue in a4edc2c9fa35a763fc5f4c9cf6383096a13a9cf6.

clang/lib/AST/ExprConstant.cpp

if (LHSTy->isMemberPointerType()) {
     if (!EvaluateMemberPointer(E->getRHS(), RHSValue, Info) || !LHSOK)
       return false;
 
     // If either operand is a pointer to a weak function, the comparison is not
     // constant.
     if (LHSValue.getDecl() && LHSValue.getDecl()->isWeak()) {
       Info.FFDiag(E, diag::note_constexpr_mem_pointer_weak_comparison)
           << LHSValue.getDecl();
-      return true;
+      return false;
     }
     if (RHSValue.getDecl() && RHSValue.getDecl()->isWeak()) {
       Info.FFDiag(E, diag::note_constexpr_mem_pointer_weak_comparison)
           << RHSValue.getDecl();
-      return true;
+      return false;
     }
 
     // C++11 [expr.eq]p2:
     //   If both operands are null, they compare equal. Otherwise if only one is
     //   null, they compare unequal.
     if (!LHSValue.getDecl() || !RHSValue.getDecl()) {
       bool Equal = !LHSValue.getDecl() && !RHSValue.getDecl();
       return Success(Equal ? CmpResult::Equal : CmpResult::Unequal, E);

clang/test/SemaCXX/crash-lambda-weak-attr.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2b %s
+
+struct Weak {
+    [[gnu::weak]]void weak_method();
+};
+static_assert([](){ return &Weak::weak_method != nullptr; }()); // expected-error {{static assertion expression is not an integral constant expression}} \
+                                                                // expected-note {{comparison against pointer to weak member 'Weak::weak_method' can only be performed at runtime}} \
+                                                                // expected-note {{in call to}}
+